Dr. Sumit Kumar (Dept Physics, RHUL)

The mechanical properties of amorphous materials (glasses) at low temperatures are dominated by effects of low energy excitations that are thought to be atomic-scale tunneling two level systems (TTLS). In nanometerscale glass samples, the temperature dependence of the sound speed and dissipation is modified relative to that of bulk glass samples. In addition to this size effect, the usual presence of a polycrystalline metal in nanomechanical resonators leads to a further departure from the well-studied behavior of insulating bulk glass. The recent field of cavity optomechanics has paved the way for sensitive detection of the position of nanomechanical resonators and also control of its mechanical characteristics. The thermal motion measurement of 50  x 300  x 100  nanomechanical string coupled to Nb superconducting microwave cavity using cavity optomechanical techniques will be discussed. The measurement of thermal motion of the nanobeam below 𝑇 < 200 mK is marred by the anomalous force noise seen in the output power from the cavity which is not consistent with the optomechanical theory, where 𝑇 is the temperature of the sample. We will show a detailed analysis of the statistics of the anomalous force noise called “spikes” and will try to give a plausible reason for the same. Further many amplifiers and notch filters can be made from optomechanical systems. They rely on optomechanically induced transparency (OMIT) and absorption (OMIA). We will investigate our results on OMIT and OMIA covering a large parameter space than has been explored in previous works.  We then report a dual chip optomechanical measurement technique used to characterize non-metallized amorphous SiN strings at low temperatures. A harp consisting of SiN strings of width 350  and lengths 40 to 80  is coupled to an Al superconducting microwave cavity on a separate chip. The strings are driven dielectrically and their motion is detected via its modulation of the microwave resonance frequency.

Fig. caption: Dual chip cavity optomechanics: Superconducting microwave cavity chip and chip with SiN nano beam